We will examine the pulmonary hemodynamic responses to various pulmonary and systemic stimuli and how these responses are modified by superimposition of one upon the other. The pulmonary stimuli include changes in alveolar gas composition and transpulmonary and vascular pressures. Systemic stimuli include systemic hypotension, elevated intracranial pressure and others which have been associated with the development of pulmonary insufficiency to which pulmonary vasomotor responses may contribute. The experiments will be carried out using isolated cat lungs and dog lung lobes perfused in situ. In these preparations, nervous, hormonal and local vasomotor influences and mechanical influences can be examined separately or in concert, allowing identification of the mechanisms involved. Hemodynamic responses will be identified using modified versions of the low viscosity bolus technique for determining the distribution of vascular resistance and ether and dye dilution techniques for determining the distribution of vascular volumes and compliances. A mathematical model which describes the pulmonary circulation will be developed to aid in the interpretation of experimental data. The results will help us to evaluate the role of pulmonary vasomotor responses in the regulation of the pulmonary circulation during physiologic and pathophysiologic conditions.